Huntington's disease (HD) is an autosomal dominant inherited neurodegenerative disorder that is characterized by progressive motor dysfunction, emotional disturbances, dementia, and weight loss. There currently is no treatment for delaying the onset of the disease or for slowing the progression of HD. Management of HD is focused on symptom reduction, and the only drug approved by the FDA is tetrabenazine, which is indicated to suppress involuntary movements (chorea) but does not slow the disease progression. The disease is caused by an elongated CAG trinucleotide repeat expansion located within exon 1 of the IT-15 gene encoding huntingtin, a 350-kDa protein of unknown function. The CAG repeat is translated into a polyglutamine (polyQ) stretch. In HD patients, huntingtin is expressed with 38-180 glutamine residues, whereas in healthy individuals the protein is synthesized with 8-37 glutamine residues. The accumulation of ubiquitinated polyQ-containing huntingtin aggregates in neuronal inclusions causes the death of affected neurons and has emerged as a pathologic hallmark of HD Inhibiting polyQ aggregation has exhibited some efficacy in vivo in both Drosophila and mouse models of HD, which suggests that inhibition of neuronal polyQ aggregation might be therapeutic in HD patients.
Sirtuins comprise a family of protein deacetylase enzymes that have been shown to impact longevity in a number of eukaryotic species. The role for sirtuins as therapeutic targets in age-dependent neurodegenerative disorders has recently emerged, primarily as a result of their functions as regulators of metabolism and subsequent effects on longevity. SIRT2, one of the seven human sirtuins so far identified, belongs to the class III histone deacetylases (HDAC), which couple hydrolysis of acetyllysine residues with NAD hydrolysis. Known substrates of SIRT2 include α-tubulin, FOXO1, FOXO3a, P523, and histones 3 and 4. (See, e.g., Taylor, D. M.; Maxwell, M. M.; Luthi-Carter, R.; Kazantsev, A. G. Biological and potential therapeutic roles of sirtuin deacetylases. Cell Mol. Life Sci. 2008, 65, 24, 4000 4018; and North, B. J.; Marshall, B. L.; Bona, M. T.; Denu, J. M.; Verdin, E. The human Sir2 ortholog, SIRT2, is an NAD+-dependent tubulin deacetylase. Mol. Cell 2003, 11, 2, 437 444.) SIRT2 level is sharply increased during neurodevelopment, remains strikingly high in mature brain, and accumulates in the aged CNS. Chemical inhibition of SIRT2 changes protein inclusion body characteristics and increases neuronal survival in animal models of Parkinson's disease. It also achieves neuroprotection in cellular and invertebrate models of HD and leads to increased acetylation of its substrate α-tubulin, a major component of microtubules, subsequently leading to neuroprotective transcriptional modulation of metabolic pathways and to reduction of polyQ inclusions in HD primary neurons by decreasing sterol biosynthesis.
Cell-based highthroughput and in silico screening identified small-molecule sulfobenzoic acid derived inhibitors of polyQ aggregation and SIRT2 (FIG. 1). C2-8 inhibited polyQ aggregation in brain slices from R6/2 HD transgenic mice at sub-micromolar concentrations, reversed the effects of neurodegeneration in a fruit fly model of HD, crossed the blood brain barrier following oral administration, inhibited huntingtin (htt) aggregation in vivo in the striatum by 25-30% in R6/2 mice, effectively suppressed aggregation in a cell-based model with an EC50=50 nM, significantly reduced striatal neuron loss in R6/2 mice and improved their clinical phenotype, markedly reduced htt aggregation by 50% in the 140 CAG knock-in model of HD, which is much more analogous to human HD, and was not toxic in acute and chronic tolerability studies. (See, e.g., Chopra, V.; Fox, J. H.; Lieberman, G.; Dorsey, K.; Matson, W.; Waldmeier, P.; Housman, D. E.; Kazantsev, A.; Young, A. B.; Hersch, S. A small-molecule therapeutic lead for Huntington's disease: preclinical pharmacology and efficacy of C2-8 in the R6/2 transgenic mouse. Proc. Natl. Acad. Sci. USA 2007, 104, 42, 16685-16689.) AK-1 was neuroprotective in C. elegans and Drosophila HD models and inhibited polyQ aggregation in primary rat striatal neurons expressing the mutant huntingtin fragment delivered by a lentivirus and selectively inhibited SIRT2 at low micromolar concentrations (IC50=12.5 μM). (See, e.g., Taylor, D. M.; Balabadra, U.; Xiang, Z.; Woodman, B.; Meade, S.; Amore, A.; Maxwell, M. M.; Reeves, S.; Bates, G. P.; Luthi-Carter, R.; Lowden, P. A.; Kazantsev, A. G. A brain-permeable small molecule reduces neuronal cholesterol by inhibiting activity of sirtuin 2 deacetylase. ACS Chem. Biol. 2011, 6, 6, 540-546.) In silico screening of a ChemBridge diverse library identified AK-7, the first brain-permeable selective SIRT2 inhibitor (IC50=15.5 μM) that reduces cholesterol in neuronal models and protects an in vitro model of HD. (Id.)
Notwithstanding the excellent pharmacologic properties of such sulfobenzoic acid derivatives, further lead optimization of the scaffold is required for CNS drug development to achieve increased potency and metabolic stability, en route to preclinical efficacy testing in HD. For instance, C2-8, AK-1, and AK-7 are characterized by the low aqueous solubility, high clogP, and low plasma and microsomal metabolic stability. As a result, there remains an on-going need for medicinal chemistry modification to, for instance: (1) enhance potency and favorable pharmacokinetic properties; (2) enhance the water solubility (lower the log P) and lower the molecular weight; (3) probe chemical space to define a structure activity relationship (SAR).